VAN 211: Veterinary Histology and Embryology (2+2)

GENITAL SYSTEM

• The genital organs are undifferentiated when they first appear (indifferent period), either by gross or microscopic appearance and hence during this period the term gonad is applied to them.
• The gonad appears within a thickening called the genital ridge on the medial aspect the urino-genital ridge.
• The ventral surface of the genital ridge shows rapid proliferation of the mesothelium which becomes thickened and many layered. The gonad consists of a superficial germinal epithelium and an internal epithelial blastemal mass.
• The primordial germ cells from the yolk sac endoderm migrate by way of the dorsal mesentery of the gut and locate themselves in the epithelium of the genital ridge.
• In the male, genital glands as they increase in size, shorten into more compact organs and begin to show branched and anastomosing cords of cells called the testes cords. These arise from the superficial germinal epithelium which dips into the interior of the mesenchyme forming cords of cells. These cords differentiate into the tubuli contorti and tubuli recti.
• The original germ cells descend into the tubuli contorti and the mesenchyme forms the interstitial tissue and the mediastinum testis. When the migration of the germs cells is complete, the mesenchyme surrounding the gland forms the tunica albuginea and cuts off the germinal epithelium, which becomes mesothelium. Certain cells of the mesenchymal stroma transform themselves into large cells lying in the intertubular connective tissue forming the interstitial cells.
• In the female, the gonad does not exhibit any distinctive ovarian features until several weeks after the gonad of the male has declared itself as a testis. Then the blastemal mass shows clusters of small indifferent cells and one or more primordial germs cells. Soon a primary cortex appears beneath the germinal epithelium and a primary medulla appears internally.
• Later, the ovary enlarges by addition of a new (secondary) definitive cortex upon the original blastemal mass. In the primary medulla and cortex, the earlier ova decline, regress and are replaced by vascular fibrous stroma-the definitive medulla.
• In the definite cortex, ova and the primary follicles develop. Later vesicular (Graafian) follicles develop from the primary follicles during the active sexual cycle.
• It is to be remembered that while the primary medulla and primary cortex correspond to the male component of a gonad, the definitive cortex of the gonad is a distinctive female characteristic.